1. Field of the Invention
The present invention relates to a method and apparatus for employing delta modulation to compress ECG signals and convert them into binary pulse trains suitable for accurate recovery of the original data and, more specifically, it relates to such a system which provides error checking means for assisting with the recovery of data and also provides improved fidelity for encoded-decoded signals at low average bit rates and further provides information which can be used to detect certain heart arrhythmias (abnormal heart rhythms).
2. Description of the Prior Art
It has long been known to employ signal modulation techniques in the processing and communication of information. It has also been known to employ delta modulation to convert a continuous signal into a binary pulse pattern. See, generally, U.S. Pat. Nos. 3,868,567; 4,509,529 and 4,567,883.
It has been known to employ delta modulation in connection with medical information such as that contained within electrocardiograms. See, generally, U.S. Pat. Nos. 3,868,567; 4,509,529 and 4,567,883.
One of the problems with known delta modulation systems is that the compression ratio is not very high if the high slew rate portions of the signal in the R waves are to be preserved. Another problem is that it is very difficult to recover from lost data and errors in the bit stream such as losses which may occur during transmission. This can lead to informational gaps and DC level shifts and drifts which are highly undesirable.
In its simplest form a delta modulator generates a continuous sequence of bit samples. Each bit represents two states, that steps the output waveform up by one step size or down by one step size. If the input waveform starts to change in one direction, the delta modulator's output bit stream will contain a larger number of bit samples which represent movement of the output waveform in the same direction. In that manner the output waveform tracks the input waveform as long as the input waveform does not change too rapidly. See, for example, U.S. Pat. No. 4,466,440 for a discussion of a simple delta modulator.
Unfortunately, one of the limitations of delta modulators relates to its ability to track rapidly changing signals such as those in the electrocardiogram (ECG) R wave. It is well known that the ECG consists of relatively long periods or nearly flat potentials, i.e. the baseline, with periodic rapid excursions from the baseline with very high slew rate, the R wave. Conventional delta modulators are unsuitable for tracking such a waveform if the objective is to provide a bit stream from which the original signal can be reconstructed such as for data transmission. The reasons for this can be readily appreciated as the delta modulator has two opposing constraints. In order to track small changes in the baseline period, the step size must be kept small, but a small step will not track the rapid changes in the R wave unless the sample rate is extremely high. At low clock rates the delta modulator is said to be "slew rate limited".
U.S. Pat. No. 4,509,529 discusses event detection by production of a sequence of consecutive ones from a data modulator. It states that a basic problem with such a slew rate limited delta modulator is that the bit sample sequence generated by the device is not a true digital representation of the analog input and it does not allow a true replica of the input signal to be reconstructed from the bit samples. In this patent it is suggested to increase the clock rate to 8 KHz in order to allow more accurate tracking of the input signal. However, for data transmission, a bit rate of 8000 bits/sec. is far too high to be practical.
It has been suggested to solve the slew limited response problem by changing the step size in response to the history of the signal. This is commonly known as continually variable slope delta modulation or CVSD. CVSD techniques are described in Motorola application note for the MC3417-MC3418 integrated circuits and U.S. Pat. No. 4,567,883. The basic idea is to change the slope or amount of change that the reconstructed signal can take during one bit period in response to the past history of the bit stream. For example, if 3 ones occur in a row, a signal is passed to an integrator whose output increases the step size.
CVSD techniques, however, have disadvantages. One concern is that of overshoot. After tracking a rapidly changing waveform such as an R wave, the step size or slope has been increased to keep up with the high slew segments. When these high slews stop suddenly at the end of the R wave, the delta modulator is "stuck" in high step mode for a short period of time, and must continue producing samples at high slew rates until the slope determining integrator can ramp back down again. This process can take many clock cycles and leads to overshoot or ringing after an R wave as shown in FIG. 1. FIG. 1(a) shows an original wavefrom with ST depression and FIG. 1(b) shows the reconstructed waveform with distorted ST segment. This overshoot can distort the critical ST segment in the reconstructed ECG waveform. Conventional delta modulators exhibit additional shortcomings. As the delta modulator reconstructs the signal as a series of changes or deltas added to some initial value, a typical delta modulator cannot accurately transmit a DC level. In case of error when some of the data lists are lost due to losses in a transmission medium, the operational DC level is lost and the signal may shift to an undesirable DC level which cannot be processed by subsequent circuitry due to this error induced offset voltage. Conventional delta modulators suffer from both shortcomings; i.e., the inability to track R waves if the sampling clock rate is low, and persistent level shifts in case of lost data or inaccurate reconstruction.
There remains, therefore, a need for providing improved means of efficiently using delta modulation concepts in medical as well as other environments.